Update on the debugging:I looked at flywheel today and it is fine, no damage to anything including key.Also tried again spark plug and it is producing sparks fine, I can see it. Spark plug is almost new by the way.Looked at both sides of carb and they both clean, no debris or anything and no damage to membranes or diaphragms and spring with needle looks fine. Fuel is definitely coming from tank, but probably not coming out of it or something else is not working correctly.I'm running out of time with milling my cedar, so I will take it to a dealer tomorrow and see what they will tell me.

.......The only other thing that comes into play, is timing. If your compression is good, pulling the flywheel would let you see if the key has sheared.

yes! The first thing I would check in such a situation. A moving flywheel that has caused a key to shear would shift forward. This is easily verified if the spark plug is removed and a pencil or screwdriver is inserted onto the piston top and the engine is turned over slowly (anti Clockwise at the flywheel)m When the N and S on the flywheel are aligned with the poles of the ignition module, if the key is sheared the piston will be nowhere close to TDC

If the key is not sheared and everything else looks OK, the usual cause on a 066/660/046/460 is the polymer flywheel going out of timing due to a twisted base.This can be verified only by having another good flywheel to compare with.Place both flywheels one above the other with N and S of both aligned.Then examine the centers if the key grooves are not aligned, it verifies a twisted flywheel.Nothing can be done other than a flywheel replacement.

I can't remember where I read it but there is a very good article on why premium fuel and proper mix out there. Bottom line, octane is not about power, it is about controlled combustion vs detonation. Detonation creates very nasty internal forces on the engine. My manuals call for premium so I will pay the $.40 extra a gallon for it. Also, if memory serves me, more oil in the mix drops the octane rating so that extra oil may burn your piston in the long run too

OP, hope you get your big saw sorted out!!

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Update from the dealer:He disassembled the engine and found what piston was scored and cylinder has some damage.From his point of view it was overheated due to hi revs or not enought lubrication or both.I should have done it myself, but I hopped it is some other issue, not piston.I will rebuild this one (I hope it is not difficult (never did it before)), but I don't have time to do it now, so I need to buy another one in order to finish my milling. I only need 2 cuts to finish my last log.I can buy new MS661 (power head only) for 1025$US or 1300$CAD or I can buy a built one from ground up using stihl and aftermarket parts MS660 for 590$US or 750$CAD. 3 months warranty

So my dielema is which one to buy: 660 or new 661?Thanks a lot for all replies, I've learned a lot.

Ask your dealer if he will do it with after market parts and vacuum and pressure test AFTER to make sure the every thing is sealed . You may have been out of adjustment or it could be seals . Let a pro do it . I tried and melted down a 272 three times . turned out someone scored a case with a screwdriver getting a seal out .

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